The present invention relates to a nondispersive infrared gas analyzer. Such gas analyzers are used, for instance, for detecting pollutants such as carbon monoxide in the air of interior rooms or in the exhaust gases of automobiles, etc.
Gas analyzers of the type to which the invention pertains are, for example, constructed to include an infrared measuring beam path and a reference beam path; and a chopper, or the like, modulates these radiation beams. Each of the beam paths includes a cell containing or being passed through by a measuring gas; but the two cells are of different lengths. The radiation beams are separately detected by receiver cells containing the particular component whose concentration in the measuring gas is to be detected. The two receiver cells are interconnected by a differential pressure sensor which includes a capacitor with a deflective membrane electrode.
A gas analyzer of the type described above is, for example, disclosed in our U.S. Pat. 4,281,248, issued July 28, 1981 (see also German printed patent application No. 28 27 345). This patent deals with a particular approach to reduce the so-called cross sensitivity of such an analyzer which arises when a particular component in the measuring gas has an absorption band overlapping an absorption band of the component to be detected. In accordance with that patent, a movable interference filter is partially inserted in the measuring beam path, affecting but a portion thereof. The present invention is related to an alternative approach which is believed to be more effective.
It should be mentioned that in our U.S. Pat. No. 4,281,248 we have suggested to include radiation-absorbing elements in the receiver cell of that radiation path which contains the shorter one of the two cells being flown through by the measuring gas. The same receiver or detection cell is internally provided with black walls; the other one has gilted walls. This particular arrangement reduces any interference. The present invention is an improvement of this earlier attempt to eliminate cross sensitivity.